She was a old friend of Gene Roddenberry's, with the exception of Patrick Stewart she was making more money than the other actors, meaning she made more than the actress she replaced.What was the issue with Diana Muldaur?
There was a feeling that Roddenberry pushed for her to be given more screen time than McFadden had been getting.
To be fair, Muldaur was a more experienced actress, with lot's a screen credits going back a couple of decades. And while some dislike the character she play, few cast doubts on her acting abilities.
Personally I liked her and would have enjoyed her being on the show longer. It (imho) might have been interesting if the show had made a policy of replacing the doctor every other year or so with a new one.
https://youtu.be/F9kDZMPg_24?t=900
Starts at 15:00 in case it doesn't go directly to it.
By the way, this is a really good website for those of you interested in television and those involved. Some excellent interviews. Rick Berman has an extensive one.